Recently, as a semiconductor material for a short-wavelength light-emitting device, a GaN-based compound semiconductor has received attention. Generally, a GaN-based compound semiconductor is formed on a substrate made of sapphire monocrystals, various oxides, or group III-V compounds by using a thin film-forming means such as a metalorganic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
A thin film made of a GaN-based compound semiconductor has the characteristic of small current diffusion in an in-plane direction of the thin film. Further, a p-type GaN-based compound semiconductor has the characteristic of a high resistivity in comparison to an n-type GaN-based compound semiconductor. Due to this, in the case where only a p-type electrode made of metal is laminated on the surface of the p-type GaN-based compound semiconductor layer, there is almost no spread of current in the in-plane direction of the p-type semiconductor layer. Accordingly, for example, if a laminated semiconductor layer having an LED structure comprised of an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer is formed as a semiconductor light-emitting device using the GaN-based compound semiconductor, and also a p-type electrode is formed on the p-type semiconductor layer of an uppermost portion, then only a portion of the light-emitting layer that is positioned just below the p-type electrode emits light.
As a method of extracting the emitted light, which occurs just below the p-type electrode, out of the semiconductor light-emitting device, there is a method of using the transparent p-type electrode, transmitting the light occurring just below this p-type electrode through the p-type electrode, and extract the light out of the semiconductor light-emitting device. As the transparent p-type electrode, a method is known that uses a conductive metal oxide such as ITO or a metal thin film of about several tens of nanometers.
For example, Patent Literature 1 discloses a method that uses a metal thin film of about several tens of nanometers. However, the transparent electrode comprised of metal oxide such as ITO or the ohmic electrode comprised of a metal thin film of about several tens of nanometers has a problem of a low strength of the electrode itself.
In order to improve the strength of the electrode itself, there is the example in which a bonding pad electrode, which is made of a metal material and has a specified thickness, is arranged on a transparent electrode made of metal oxide such as ITO or a p-type electrode made of a metal thin film of about several tens of nanometers. However, since this bonding pad electrode has no transparency and shields light, there was a problem in that it was not possible to extract light out of the semiconductor light-emitting device.
In order to solve this problem, for example, Patent Literature 2 discloses a method of laminating a bonding pad electrode formed of a reflection film, such as Ag, Al, on a p-type electrode. By forming the boding pad electrode from the reflection film, it is possible that the emitted light that has transmitted the p-type electrode is reflected to the inside of the light-emitting device by the bonding pad electrode and the reflected light is extracted from a portion other than a bonding pad electrode forming area to the outside of the light-emitting device.